Solvent-based coating compositions used in automotive finishes or industrial paints typically include a polymeric resin base in which are dispersed flake-like pigments, such as metal flakes, that create a color effect in the coating composition such as metallic luster or goniochromism. The metal flakes tend to align parallel to the surface of the coated substrate, thereby presenting a flat, reflective surface which gives a desired appearance to the coating when the paint dries or is baked.
Tough abrasion resistant coatings can be produced by powder coating methods. However, alignment of flake-like pigments in powder coating compositions to achieve the color effects similar to those achievable in solvent-based coating compositions is problematic. Powder coating compositions are commonly applied through electrostatic spray coating. In the electrostatic spray process, components of the coating composition (including a polymeric resin and additives) are ground to produce powder particles which are dispersed in an air stream. The dispersed particles are subjected to a high voltage field in which the particles pick up an electrostatic charge. The charged particles are attracted to a charged substrate resulting in a fine layer of powder on the substrate. The coated substrate is heated to a temperature sufficient to melt the powder coating and to cause it to flow and provide a smooth, even finish.
Metal flakes can be included in a coating powder by admixing the metal flakes with the resin, as well as flow-control agents, curing agents, pigments, fillers, etc., prior to melt-compounding of the ingredients. However, during grinding of the composition to produce a coating powder, the flakes are often significantly fragmented, and the finish that results from such a coating powder has a dull, grey appearance.
Attempts to enhance the appearance of metallic finish have focused on methods of admixing the flake-like pigment with the polymeric coating powder particulates. However, in the powder coating process, such an admixture often results in inconsistent coatings. In the electrostatic coating process, the coating powder is sprayed through a gun where a high voltage corona discharge is used to establish an ionized field. As the powder particles pass through the ionized field, they become charged and are attracted to the substrate, which is usually a metallic article to be coated. Powder that is not deposited on the substrate is collected in a reclamation system and returned for admixture with the virgin material for reapplication. When metal flakes are merely admixed with the polymeric coating particulates, the flakes are not charged to the same extent or deposited at the same rate as the polymeric coating powder particulates. Over a period of time during which overspray is continuously reclaimed and admixed with virgin material, the metallic flake pigment becomes more concentrated in the powder being sprayed, changing the appearance of the resulting finish from the finish which resulted from spray at the start of the run. Eventually, the increasing concentration of metal flakes may interfere with the charging mechanism. If the powder in the ionized field becomes too conductive, the electrostatic gun may “short out” or result in an explosion.
One solution to this problem of producing powder coating compositions that contain flake-like pigments has been to fuse or bond the flake-like pigments to particles of the base resin of the coating composition. By adhering the flake-like pigments to the resin particulates, the composition does not change significantly over time in a process in which overspray coating powder is reclaimed and reintroduced. Accordingly, the resulting finishes are more uniform over time.
Bonding of flake-like pigments to resin particles can be achieved by high shear mixing. However, many flake-like pigments are sensitive to fragmentation in high shear environments whereby the flake shape is damaged or destroyed, such that the resulting powder coating compositions yield low quality finishes.
An alternative method of bonding that has achieved some commercial success involves controlled heating of the flake-like pigment and resin mixture with low or medium shear mixing. According to this method, the powder composition is heated just enough to slightly soften the resin particles so that the flake-like pigment particles will adhere thereto. While elevated temperatures have been demonstrated to enhance bonding of flake-like pigments to resin particles, this conventional practice dictates that the bonding temperature be no more than 2° F. higher than the TG of the base resin. By controlling the bonding temperature to a maximum of 2° F. higher than the TG of the base resin, it has been thought that the resin begins to soften yet remains particulate (not melted) so that bonding can occur with the particles of the flake-like pigment without losing the particulate form of the resin.
While this blanket approach to bonding based solely on the TG of the base resin can be used with some success, it does not account for significant differences in the total composition of the powder coating, i.e., the resin, flake-like pigment, fillers, etc. The potential heat intake of a composition is determined not only by the base resin but also by all the components thereof. As such, the control of bonding processes based solely on the TG of base resin is not necessarily appropriate for all powder coating compositions.
Accordingly, a need remains for a method of producing coating powders which result in a lustrous metallic finish that is consistent over time when using various base resins, flake-like pigments and fillers in a process in which overspray particles are reclaimed and returned.